This invention relates to reverse osmosis membranes.
Removal of materials dissolved or dispersed in a solution by separation of these materials from the dispersing medium or solvent utilizing reverse osmosis membranes is well known. The membranes used for reverse osmosis are selectively permeable to certain components of the mixture to be separated. Generally, water is the component to which such membranes are especially permeable. The aqueous feed solution is conveniently brought in contact with one surface of the reverse osmosis membrane under pressure so as to effect permeation of water through the membrane.
The performance of the reverse osmosis membrane has a critical effect on the efficiency and efficacy of reverse osmosis membrane processes. Advantageously, a reverse osmosis membrane should combine high rejection of solutes with a high water flux and should display resistance to deterioration by hydrolysis or oxidation.
Polyamide membranes have been widely employed in reverse osmosis processes. U.S. Pat. No. 4,277,344 describes a variety of membranes having a polyamide discriminating layer. Thin film composite membranes having a crosslinked polyamide discriminating layer have proven particularly useful in reverse osmosis processes. Such membranes are further illustrated in U.S. Pat. Nos. 4,520,044 and 4,606,943.
U.S. Pat. No. 3,551,331 describes a process for modifying the permeability of a substantially linear aliphatic polyamide membrane. In this process, the polyamide membrane is treated with a protonic acid, lyotropic salt or a Lewis acid.
U.S. patent application Ser. No. 790,287, filed Oct. 22, 1985 now U.S. Pat. No. 4,704,324, issued Nov. 3, 1987, describes membranes prepared by reaction of a nucleophilic first compound with a second compound bearing reactive onium groups.
Although the commercial polyamide membranes can display an excellent combination of water flux, salt rejection and chemical stability, membranes having improved performance characteristics are still being actively sought. Further, processes for enhancing the flux of membranes which have become fouled during use are desired.